Two US citizens killed in Brussels bombings

US Secretary of State John Kerry participates in wreath-laying ceremony at Brussels Airport in Brussels, Belgium.

Secretary of State John Kerry today pledged further US help to Belgium and the EU in tackling Islamist militant threats after Tuesday's suicide bombings in Brussels and a US official said two Americans were among the 31 dead.

Kerry also dismissed criticism of Belgium's failure to track Islamic State militants before the attacks as a tendency to "jump to conclusions", but said there was room for improving anti-terrorism cooperation among European Union nations.

"The United States is praying and grieving with you for the loved ones of those cruelly taken from us, including Americans, and for the many who were injured in these despicable attacks," Kerry said after meetings with Belgian Prime Minister Charles Michel, Foreign Minister Didier Reynders and Jean-Claude Juncker, head of the EU executive body based in Brussels.

"We will continue to provide any assistance necessary in investigating these heinous acts of terrorism and bringing those responsible to justice ... We offered today anything they need and they are already taking us up on (it)," Kerry said.

"We will specifically work with them to help in the coordination of the flow of information."

He added that EU governments had been working over the past year to better exchange counter-terrorism information. "There is room for additional coordination," he said.

Belgium has drawn international criticism for security lapses in the run-up to the bombings in Brussels airport and a metro station which, in addition to the 31 dead, injured around 270 people. The US official declined to name the Americans killed until their next of kin had been notified.

Kerry took issue with the criticism. "This carping four days later is a little frantic and inappropriate. People are jumping to conclusions, they are looking for things that sometimes just are difficult to analyze in the immediate aftermath."

He told reporters he did not know whether opportunities had been missed by Belgian authorities in tracking Islamic State militants, but added: "This government has been in place for a year and it has moved very forcefully to deal with terrorism."

Kerry said the United States was working with the Belgian government before the attacks and had a "foreign fighter surge team" in Brussels in February to work on specific counter-terrorism efforts.

There were also a number of counter-terrorism initiatives already scheduled to take place in Brussels before the attacks occurred to help detect suspected jihadists, he said.